The state machine for 3-button emulation defers some button events

until a 20ms select(2) timeout occurs, but if there is a continuous
stream of movement events, button events can be delayed indefinitely
because the select never has to wait long enough for a timeout.
The delay and mouse event reordering that result are very noticable
and sometimes quite frustrating when dragging windows etc. in X.

Add a simple mechanism that avoids this re-ordering. While a button
event is deferred, we discard up to 3 movement events to allow for
mouse jitter. If more movement events occur, then we immediately
timeout the deferred button event and let the movement proceed.
This change only affects the 3-button emulation case.

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